Frequency converter



June 27, 1944. J. L. Rb wscH 2,352,451

FREQQE-IEY CONVERTER Filed Feb. 7. 1942 INVENTOR Q J. L. ROEM/SCH Arrow/Ex Patented June 27, 1944 UNITED STATES PATENT OFFICE q FREQUENCY CONVERTER Joseph L. Roemisch, Montclair; N.-J., assignor to Western Electric Company; Incorporated, l New York, N'. Y., a corporation o' New York Application February '1, 194 2,\ S erial No.,429,932

3Glaims. (Cl. 25036) This invention relates to frequency converters and especially to what has frequently been denominated a self-synchronizing system wherein awaveof a given frequency from a coercing oscillator coerces an oscillator to cause it to opere ate in locked synchronism with the coercingoscillator and to generate a wave having a frequency bearing a commensurable relation to th frequency of the coercing Wave. Theabove statement as to the necessary-commensurable frequency relation between; the two waves concerned was used to indicate the possible breadth and generalityof the invention.

That is, although the invention may be-used, and

More particularly, the inventioncomprises a self-synchronizing system. as above in which the coerced oscillator is of the negative transconductance tube type and especially one inwhich thereis a strategically positioned and electrically valued impedance: adapted to make said oscillator more easily coercible without detriment to other functions which make the measure of an efficient and'accurate self-synchronizing system. It is an object of, theinvention to achieve self-synchronization by -.the use of a negative transconductance tube oscillator as the coerced oscillator. A It is a moreparticular object .of the. invention, consistenly with the above brief characterization, to so electrically condition a negative transconductance tube oscillator-to make it moreeasily coercible. consistently withv the satisfaction of other desirable conditions inherent in-the=operation of efficient self-synchronizing .systems.-.

It is a further object of the invention to provide an oscillator that is readilycoercible-over a wide range of the coercing frequency andfur nishing anoutput signal of essentially sinusoidal wave-shape. 1

It is a still further: object, ofthe invention to gree of stability and purity of output wave shape ing: wherein:

and, yet being capable of a high degree of coercion by a coercive-signal much. smaller in magnitudethanthe coerced signal. v

It isa feature of the invention. thatthecin cuit. arrangement is limited, as regards, coercion, by the-use of certain circuit constantsfor in ductance and capacitance to coercion atonly the approximate antiresonantv frequency of the inductance-capacitance circuit, v

The above objects can only berealized bye circuit, utilizing. a discharge tube containing .elements that will permit its ready funnctioning as anegativetransconductance oscillator of thedirect coupledtype and conditioned by means ofa suitable resistor R,- to be' 'described, in the anode lead ofthe discharge tube. r v

fIfhe ,organization of the invention. is analternative in; its class of self-synchronizing systems. For instance it achieves in general the same ultimate result, and in somewhat the samemanner, as; the. organization of the same classmin which acontrolled multivibrator is.used.j However, the present organization has. an advantage thereover;- in that it requires the use, of only one elec-v tric discharge tube as against'two tubes required by amultivibrator. There are other advantages over this, and other alternatives: in the same c1ass,-some of-which arespecific per se to the particular type of coerced oscillator used, namely the-negative jtransconductance' tube oscillator, besides :others which illustrate the particular adaptability of this type of coerced oscillator in the i-self-synchronizi-ng organization, but which are not suggested from the-characteristics of this type of oscillator when used as an independent source of oscillations. It-is believedthatapplicant is the first to have appreciated these advantages and tohave discovered a means for realizing them in a practical way.- The advantages-herein contemplation will. more appropriately-be pointed. out in connection with the detailed-description that follows. i. z A more full description of the-invention appears hereinafter in connection .Withthe draw- Fig. 1 illustratea'in large part-diagrammatically, a specific embodiment of the, invention;

cingliasource is shown diagrammatically at l;-

The remaining part of the circuit, that is, the

coerced oscillator, is, largely because of negative transconductance tube 4, a negative transconductance tube oscillator, generally of the type disclosed in United States patent to Herold A negative transconductance tube, as exempli fled herein, and in the Herold references, is a frequency is determined in some way by a resonant circuit.

The portion of the circuit of the coerced oscillator, as so far described, because of the included negative transconductance tube 4, well lends itself to the production of self-sustained oscillations. The coincidence of potential phase on the control grid or electrode 5 and anode I in fact makes it more easy to evolve a self-contained oscillator from the circuit than from the corresponding elements of the conventional oscillator tube, since a feedback in proper phase from the anode l to control electrode 5 may be achieved tube in the operation of which an increase of potential on the control electrode results in a corresponding increase of potential on the anode, exactly the reverse of the operation ofa con- T ventional tube in which an increase of. control electrode potential causes an increase of current in the anode circuit resulting in an increased drop and hence adecrease of the anode potential. As stated in a complementary way, a negative transconductance tube is a tube in the oper'- ation of which anincrease of potential on the control electrode results in a corresponding decrease, instead of an increase, in the anodecurrent. In the instant case, and the parallel with the above Herold disclosures should be at once apparent, an increase of potential on the con trol grid5 withrespect to the cathode fi results by a direct connection therebetween, although in :this instance a capacitor l4 having a low react- ,ance for the frequency of the LC circuit I3 is used to block off from control grid 5 the direct current potential applied to anode 1 since control grid 5 must be negatively charged. The

in an increase of potential on the anode I. Usually, as .in' the instant case, a pentode tube is used,v comprising, additionally-to the above, the auxiliary anode 8 and the synchronizing or injection grid.9. The grid 9 is not of great importance in the circuit ofthis figure, but the grid 1 here referred .to as the anodef and, the suppressorgrid 5 here referred to as' the controlgrid are of great importance as being essentialin the negative transconductance tube.

Inparticular the potential on the anode I should be positive and relatively large as compared with the positive potential on auxiliary anode 8, the potential on' the control grid 5jbeing negative and small. The potentials indicated in Fig. "1' for electrodes 5, I and 8 would be very satisfactory.

Any of pentode tubes 6SJ7, 6SK'7,'6D6, 606, 57, and 58, togetherwith others, would be satisfactory. The"potentia1 for the anode 1 and auxiliary anode B-are derived from direct potential source ID, the indicated potential of 105 volts I for theanode beingintended to be with reference to the cathode,that isQbe the potential of the source 10 as a whole instead of only that portion. of it between the auxiliary anode 8 and the anode. The same statement is applicableto the'Figs. 2 and 3 modifications since, as is pointed out in the brief description of the fig ure, the species are identical except for the specific means for introducing the coercing wave.

Obviously alternative'means a choice of which are well known in' the art, could be used to obtain thesepotentials. The negative potential for the control grid is derived from source l2 through resistance l I. The supply current for the anode I is fed through resonant frequency determining circuit l3 and resistor'R.

Resistor R is the most important feature of the system of the invention, since it is the element which enables the negative transconductance tube oscillator, the other elements of which have long been known,to be coerced by oscillator I with results greatly superior in degree of synchronizing control to that of any previously known oscillator or frequency generator whose negative transconductance tube oscillator is completed by adding said connection including the capacitor l4 between said elements I and 5 and the resonant tuned circuit I3, which iseifectively in circuit between said anode l and the cathode 6 and is adapted to determine the characteristic frequency of said oscillator. Because of the negative transconductance tube arrangement, as above explained, oscillations may be generated, by virtue of the peculiar characteristics of the tube together with the use of the frequency de termining circuit [3, having a high degree of inherent stability and in addition a very low degree of harmonic content.

The self-synchronizing system of Fig; 1 is made complete by the connection of the coercing oscillator l as shown to the coerced negative transconductance tube oscillator. As shown, this coupling is to the anode of the coerced oscillator. However, the coupling could be made to other elements of the coerced oscillator tube without detriment to the operation of the principle of the invention. In fact, the circuit of Fig. 2, to be described later, illustrates an alternative connection. No specific means is shown for taking off power from the coerced, that is, controlled, oscillator. However, this'is not important since the power take-off connection may be made almost impartially to the various tube elements. Generally such power is taken ofif across resistor II. It is noted that in the above referred to Herold Patent 2,217,748, October 15, 1940, an additional anode, coupled to the oscilla'tor'elctrodes proper only through the electron stream is used. This expedient could equally well 'be used in the circuit of applicants invention.

The operation of a negative transconductance tube oscillator maybe described briefly as follows: As above described the distinguishing characteristic of a negative transconductance tube, and the operation to be expected in the present instance, is that the anode current decreases when the control grid potential becomes less negative, or more positive. Assuming that a pulse occurs by closure of the anode supply circuit or by any other circuit disturbance, a potential will result across resistance R and tuned circuit I3. This potential will affect the potential'of the control grid through the coupling capacitor l4 and cause it to be less negative (assuming a positive pulse). Negative resistance will thus be created and the anodecurrent will decrease until it can decrease no more at which point the tube will discharge the electric charge represented by the capacitance in the tuned circuit l3 and another sequence will start. The freetialflisimpressedfrom-source: l havingfiprecisely;

the frequency andiphase of. the wave generated by the oscillator, there will; obviously =be1-no in} teraction. Howevenxa deviation from-rfrequen'cy coincidence will result;in-:the introduction off'a new, coercing,- force; Itisnot necessaryto postwlate a relative change of .the' oscillation frequency either upwardlymr downwardly; from-the coerfc' ingfrequency nor is'it necessaryzto. explainthe precise mechanics of the operation- It is true, however, that'the coercing, force-c1- potential is always in proper phase. toeaid'the negative re sistance function that causes control. 1

. Thegrid 9, while not used inthe Fig. 1 species directly in the-coercing: action, nor is needed as apart of thenegative:transconductance tube-oscillator per se, .does provide a means for control, and this. will be iljlustratedin Fig. 2. If this grid should :become negatively :charged, obviously q no electrons-couldpass it andathe circuit-could not oscillate at.al -l.- It itrbecamepositive, more electrons could flow -to the-ranodewand. control grid, this resultinginamore favorable condition and alower negative resistance. Therefore a wave applied-to said grid, 9;:does tend to. influence the circuit by changingthenegativeresistanceof the 5.

tube. If: the negative resistance is: lowered. the circuit will oscillate morereadily and-vice versa. NOW-,';if a. resistor, like resistor R. isinserted in the; anodexlead, thecircuit: tends to become a relaxation oscillator, in'fact would. become a relaxation oscillator were it not for the high impedance oftheLC circuit I3. Ijf the'res-istance of said resistor is increased toomuchzthecircuit will, cease oscillating... The point" of maximum coercion-is just before oscillationscease. This pointsto one: of the necessar conditions for emcient-operation to be discussed a little later.

As has been stated, this resistor R is principally responsible for the unique results achieved in applicants self-synchronizing combination. The use of resistor R and its adjustment to the maximum value that will still permit the normal oscillation of the circuit at a frequency determined by circuit l3 permits the oscillator arrangement to be coerced by a coercing signal at the fundamental, subharmonic, harmonic, etc. of the coercing signal frequency to a degree far greater than that achieved by any other system wherein the coerced oscillator is one whose frequency is determined, substantially, by the reso- I? nant frequency of a circuit connected or associated therewith.

In addition this arrangement permits the coercion of an oscillator having a signal output of relatively large amplitude by a coercing signal having an amplitude much smaller than that of the coerced oscillator. All previous systems wherein the frequency of the coerced oscillator is determined by the resonant frequency of an associated coil and condenser require the coerced oscillator signal amplitude to be considerably less than that of the coercing oscillator. This results in a peculiar disadvantage in that the coercin signal amplitude must be readjusted for various control ratios. In the present arrangement the amplitude of the coercing signal of a single value is approximately optimum for all control ratios. This feature contributes to the practicability and desirability of the system when a single coerced tube is used for several control ratios merely by ill) changing the' frequency; constant .of the LC" cirt |:3;'by means or suitable switches and switch; ingarrangements"well-knownto the art. It may be pointedout that the coercion of-circuits'havigna natural frequency-higherthan thecoer'cing wave frequencyand of course harmonically related is fairly simple butthe coercion of circuits with "f-requencies which are integral multiples of the natural frequency -of thecoerced circuit has been very dl-fii'cult priortothe present-invention; S'ee Proceedings of 'the'I. E. E1, page l960g -volume 18, No. 11, November 1930, pa'per byGroszkowsk-i. Thisdifiiculty isnotpresentin the operationof applicantsinventiom i -Inorder to enable resistor R to most efficiently achieve the above'results, certain electrical conditions must be satisfied; one- 0f which has a1 ready been hintedat.- The resistanceo'f resistor Rmust itself be as large as ossible short of causing the circuit to stop oscillating. Of course; as has-been stated before, circuit. I3 must'be tuned to the desired frequency which, however, must have a commensurable relation to 'thefrequency of thecoercing-wave. In the most usual case, there is an integral harmonic-relation and in general the smaller the harmonic'the more easily coercible the oscillator becomes. The Q of-th-is tuned circuit must always'be the largest possible. This means practically that the ideal situation is where Qand the'resistor R are both maximum. The usual limitation for the Q of said tuned circuit l 3 is a practical one dependent on the design of the coil comprised in it.' Since the limiting value of the resistance R is dependent on the maintaining of oscillations, said resistance cannot usuallybe made greater than about 10,000 :ohms. The values of thetube'potentials and the relations between such values, particularly the value of the bias of the control grid 5, which also should be 'as greatly negative as possible consistently .with the ability'of the circuit to maintain LC oscillations; do afl'ect the degree of control but do not affect the optimum values, as above, of Qand the resistance of element R. Of course, the relations of the tube potentials must conform with the requirements for a negative transconductance tube oscillator. R must always be in, the anode circuit in series with the LC circuit I3, although whether on the anode side or the battery side is immaterial, just so long as it is not by-passed to ground. The position shown is the desirable and the most practical one. for other reasons of convenience in circuit layout. These considerations apply as well to other species of the invention as to the Fig. 1 species here being considered.

Fig. 2 represents a modified form of the invention otherwise represented by Fig. 1. The

differences do not relate to the invention in any fundamental way. As a matter of fact these two figures actually indicate the evolution of the invention in applicantsmind. That is, Fig. 1 represents his earliest conception of it and Fig. 2 a later conception of it and a conception perhaps more mature and practical. For instance, the Fig. 2 circuit would tend to conform most nearly with modern practice of the invention and does represent very closely a practical embodiment of the invention found very effective in practice by applicant's assignee. Because the circuits are so nearly alike, duplicate reference characters have been used. The only change is the use of the synchronizing grid 9 to impress the coercing wave. That is, in Fig. l

thecoercing signalwas impressed on one of the oscillato1'-.- electrodes, grid Sybeing deemed; in that figure not :tobe an oscillator electrode and having an unimportant function-and being connected directly to the cathode as in the case of a suppressor grid in a normallyutilized pentode, whereas in the Fig. 2 circuit identically the same tube is used but this grid 9 has been separated from its direct connection to the cathode 6 and used to couple: the coercing oscillator to the coerced-oscillator; It should be noted that the resistor R, which-is theqprincipal novel feature of applicant's invention, is found inFig. 2 in the same position as in Fig. 1. It has the same function in that circuit as in the first considered circuit and the considerations adduced in connection with the treatment of the Fig. 1 circuit, affecting. said element R and associated elements,gequally govern in the present instance.- 2 4 v Fig. 3 difiers from Figs. 1 and 2 only as showing how the coercing wave may be introduced to the coerced oscillator by way of the control grid. .Since this is the only distinctive feature and sinceythelabelling is the same as in the other figures, further description is dispensed With.,-,s---

The invention has .been exemplified in three species. in the-specification above, but changes therefrom and other modifications representing new, species mayreadily suggest themselves to those skilled inthe art from a reading of this specification.;-- 1

7 What is claimed is:

-1. In a self-synchronizing system, a coercing oscillator, a coerced oscillator comprising an electric dischargerdevice provided, in like order," with a cathode, vanode, control grid and auxiliary anode,-andmeans adapted to impress a positive potential, .with respect to said cathode, on said, anode, a smaller positive potential on said auxiliary anode, and a small negative potential on said control grid, together with connecting circuits, the organization as a whole being adapted to constitute an oscillator of thenegative transconductance tube type, said coerced oscillator being adapted to generate a wave having a frequency bearing an approximate commensurable relation to :the frequency of the wave from the coercing oscillator, and means coupling said oscillators whereby said oscillators operate in locked frequency relation, said coerced oscillator also comprising a resistance in the anode lead, said resistance having the largest value and the bias on the control grid the largest negative valuethat the coerced" oscillator will tolerate without cessation--01 sinusoidal oscil-' lations while being coerced 2. The organization specified in claim 1 including a capacitance coupling means between the anode and control'grid, and a tuned circuit resonant at the desired coerced frequency in serial relation with said'anode lead resistance between the anode" and cathode, said coupling means between the coercing and'coerced o'scillators being adapted to connect said coercing oscillator between said cathode and anode.

-3. The organization specified in claim 1; in which said negative transconductance tube oscillator also comprises an auxiliary grid interposed in the electron stream between the cathode and anode, a capacitance means coupling the anode and control grid of the device, and a tuned circuit which is antiresonant at substantially the desired coerced frequency in'series with-"said anode lead resistance between said anodeand said supply source, the Q of the tuned circuit having the greatest value consistent with the production and maintenance of stableoscillations of sinusoidal waves, and said coupling means between the coercing and coerced oscillators being adapted to connect said coercing oscillator between said cathode and auxiliary grid.

' JOSEPH L. ROEMISCH, 

